Documentation: spell 'git cmd' without dash throughout

The documentation was quite inconsistent when spelling 'git cmd' if it
only refers to the program, not to some specific invocation syntax:
both 'git-cmd' and 'git cmd' spellings exist.

The current trend goes towards dashless forms, and there is precedent
in 647ac70 (git-svn.txt: stop using dash-form of commands.,
2009-07-07) to actively eliminate the dashed variants.

Replace 'git-cmd' with 'git cmd' throughout, except where git-shell,
git-cvsserver, git-upload-pack, git-receive-pack, and
git-upload-archive are concerned, because those really live in the
$PATH.
diff --git a/Documentation/git-write-tree.txt b/Documentation/git-write-tree.txt
index c8899d5..bfceaca 100644
--- a/Documentation/git-write-tree.txt
+++ b/Documentation/git-write-tree.txt
@@ -17,17 +17,17 @@
 
 The index must be in a fully merged state.
 
-Conceptually, 'git-write-tree' sync()s the current index contents
+Conceptually, 'git write-tree' sync()s the current index contents
 into a set of tree files.
 In order to have that match what is actually in your directory right
-now, you need to have done a 'git-update-index' phase before you did the
-'git-write-tree'.
+now, you need to have done a 'git update-index' phase before you did the
+'git write-tree'.
 
 
 OPTIONS
 -------
 --missing-ok::
-	Normally 'git-write-tree' ensures that the objects referenced by the
+	Normally 'git write-tree' ensures that the objects referenced by the
 	directory exist in the object database.  This option disables this
 	check.